Hal,
Congrats on the new rifle and it's a great cartridge. The original loads were 340 and 380 gr bullets and 130 gr of black. The nitro for black load was 53 gr cordite and a 440 gr short jacketed bullet. There are a couple of heavier bullets listed for the 3 1/8" cartridge up to the 570 gr bullet you mentioned, but not for the shorter case. There was not a full blown nitro express load or rifles produced for either as far as I can find. Anywho the load I've been using is 53 gr of IMR 4198, a magnum primer,13 gr of dacron and a 370 gr bullet from a modified Lyman 515141 mould. This load regulates well in my Manton double. I've tried some black powder loads but that is still a work in progress.
Have fun and let us know how it works.